Copper and its alloys are the most commonly used metals in electronic applications such as providing conductive circuit paths for printed circuit boards (PCBs). PCBs require electronic components to be attached to copper or copper alloy surface pads or through-holes by a soldering operation. Leaded components can be inserted into through-holes followed by wave soldering, or surface mount technology (SMT) components can be attached to surface pads by applying solder paste to the surface, for example by screen printing, then placing the component onto paste followed by reflow soldering. For SMT assembly operations a minimum of two reflow cycles are required in order to attach components to both the front and back of the PCB. For more complex assemblies additional reflow operations may be required to attach additional components or to carry out repair operations.
The copper surfaces of PCB pads to which components are mounted are typically coated with a protective metallic or non-metallic finish. Such protective finishes are designed to maintain good solderability by preventing the copper surface from being oxidized either during storage after PCB fabrication or during exposures to soldering temperatures.
There are several methods by which protective finishes can be applied to PCBs: by electrolytic, electroless or immersion deposition of a metal from solution, or by immersion treatment in either a solution which deposits a protective organic solderability preservative (OSP), or in a bath of molten solder alloy in a process known as Hot Air (Solder) Leveling (HAL/HASL).
OSP is considered to be a low cost SMT-compatible non-metallic surface finish method due to the excellent surface co-planarity of the coated pads. OSPs used in the current PCB industry are predominantly based on azole compounds, such as imidazoles, benzimidazoles and their derivatives.
All these N-heterocyclic compounds adsorb on copper surfaces via the formation of coordination bonds with copper atoms and have the capability to form thicker films through formation of copper-N-heterocyclic complexes. OSP film formation is preferred to be copper-specific with much lower rates of film formation on gold or other surfaces in order to prevent contamination of these substrates during the film formation process on copper. In general, OSP coating thickness is from 80-500 nm. Thinner coatings tend to lower the protectiveness against oxidization of copper surfaces, while thicker coatings tend to result in the deterioration of solderability.
The ability of OSP process to continue to evolve generation by generation to meet increasingly severe performance requirements resides in the diversity of derivatives of N-heterocyclic compounds that have been synthesized. At the present time azole compounds for OSP processes have gone through at least five generations.
An Enthone copper tarnish prevention product, ENTEK CU-56, based on benzotriazole, was first used as an OSP in the 1960's. See Soldering and Surface Mount Technology, 7(2), 6-9, 1995. The thickness of the benzotriazole film formed on copper was low, usually less than 10 nm. In addition, the decomposition temperature of the benzotrizaole-copper complex was low, i.e., around 75° C. and the protective layer typically only tolerated a single tin-lead thermal reflow cycle.
Second generation using substituted imidazoles as the active component was introduced in 1977. See U.S. Pat. No. 3,933,531. These materials formed OSP films with thicknesses above 0.2 microns but had relatively poor stability at higher temperatures.
The third generation of OSP compounds using benzimidazoles where a benzene ring is fused to the imidazole ring was introduced in 1990-1991. See U.S. Pat. No. 5,173,130. Benzimidazoles have been widely used as the main component in many commercial OSP products by a number of companies in the PCB industry. Benzimidazoles form complexes with copper very efficiently with film thicknesses which range from 10 to 100 nm. The thickness can be further increased to 500 nm to 600 nm by adding metal ions into the working solution. However, the deposition selectivity of benzimidazoles was still poor and OSP coatings formed on gold surfaces in selective electroless nickel immersion gold (ENIG) processes.
Owing to the complicated panel design technology, more reflow cycles were required to attach additional components and to carry out re-work operations. Accordingly, further improvement on thermal stability of the OSP coatings was needed. A fourth generation of OSPs was developed. These were substituted benzimidazoles, such as 2-substituted benzimidazoles, and were introduced into the industry in 1997. The thermal resistance of the OSP was greatly improved by introducing the substituted group to the benzimidazole ring. The decomposition temperature of such organic-copper complexes was significantly higher, i.e., around 350° C., resulting in extremely high film stability on copper at thicknesses in the range 100-300 nm.
Several different approaches have been used by suppliers to reduce OSP film formation on gold surfaces. In 2003 Wengenroth patented the use of benzimidazole derivatives containing pre-dip compositions which accelerate subsequent OSP film formation on copper surfaces. The accelerated film formation allowed the use of lower concentrations of active material in the main OSP bath, thus reducing the film formation on gold surfaces. Approaches based on modification to the main bath formation have also been found to be effective with Shikoku commercializing several products based on the use of an iron additive.
The shift towards lead-free soldering in the PCB assembly industry that had emerged in Japan in the 1990's was accelerated by the requirements of the 2003 European Union RoHS Directive. This forced a shift in soldering processes to lead-free alloys which require about 30° C. higher peak reflow temperatures than tin lead alloys. Driven by this increase in peak reflow temperatures, a fifth generation of OSPs was developed using aryl-phenyl imidazoles as the active components. Both the thermal stability and deposition selectivity of these OSPs was improved. Although there are now a wide variety of OSP products based on azole compounds, such as imidazoles, benzimidazoles and their derivatives, there remains a need to improve the performance of OPS compositions and methods.